Make sure your wifes machine is truly dead. Usually the fixes are simple. Sometimes just a matter of a good cleaning/oiling.
All my machines/sergers were someone elses "dead" machines.
Silnylon is already silicone impregnated. The caulk (g.e. silicone II, clear) can be used full strength to glue hems so no sewing is needed. You also thin it as you say, camp gas works too, to seam seal sewn places. BIG FIRE HAZARD!! STAY AWAY FROM SPARKS!!

You can practice on plain old coated rip/stop....and you'll get a dandy shelter from it, just a little heavier.

Regularly check your local Walmart's $2/yard bin. I've found silnylon there by the bolt and plenty of ripstop. Just keep checking.